"The GNU Project was launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system. (GNU is a recursive acronym for "GNU's Not Unix"; it is pronounced "guh-NEW".) Variants of the GNU operating system, which use the kernel Linux, are now widely used; though these systems are often referred to as "Linux", they are more accurately called GNU/Linux systems." -- http://www.gnu.org
GNU/Linux is the more accurate name for GNU software running on top of the UNIX-like Linux kernel, by Linus Torvalds. The GNU project was founded on the basis that an operating system could be free software, and that includes the kernel. However, the GNU organization by 1990 -- had yet to provide such a kernel, that would be the basis of their programs. Although they had one in progress, the Hurd which runs on top of mach, it was not nearly finished. In fact it's not even finished today as of this writeup. (I think OS X runs on top of mach). Thats where the Linux kernel enters. The Linux kernel was a private undertaking of Linus Torvalds of making a free UNIX-like kernel. Since the GNU programs had yet to have a kernel, they would be better off porting their programs to match Linux than waiting for the Hurd kernel to complete developement.
Instead of waiting for the GNU/Hurd system to complete its developement, you could now use the Linux kernel with GNU software. And since it's not on the Hurd kernel, but on the Linux kernel -- its name is: GNU/Linux.
If you are running "Linux", because of this, you would be better off saying that you are running "GNU" on "Linux" or "GNU/Linux". Unless you are a kernel monkey that is.